This invention relates to a method and means for producing a control signal for the automatic control of a process and more particularly to a method and means for producing a control signal which is removed without creating a bump in the process at a time when the control error becomes zero.
The control action which results from use of the control signal produced by this invention has some of the characteristics of proportional control action in that the control signal is normally zero when the control error is zero. However, the resulting control action also has some of the attributes of reset action in that the control signal continually increases in response to the time integral of the control error. Thus, the control signal described can be utilized in conjunction with a standard proportional control signal where the proportional control signal is added to the control signal produced by the present invention to provide a substantial equivalent to a proportional plus integral control signal with a removable integral signal.
In some industrial processes using the standard proportional plus reset control the problem of reset windup in the process control is a serious problem which has received considerable attention in the past. Various arrangements have been provided by prior art process control systems for providing reset action which is removable at an appropriate time. However, in those prior art devices the removal of the reset in itself usually caused a bump in the process by producing in the control signal a step change at the time the reset action was removed.
One process which can advantageously utilize the present invention is the cement making process which involves a cement kiln. More specifically, the temperature control of certain portions of the cememt kiln which involve a reaction which is normally exothermic are complicated by characteristics of the kiln process which cause the process to lose its exothermic nature under certain conditions. It will be evident that when the process is no longer exothermic in the area in which the temperature is being controlled the effect of a normal proportional and reset controller would be to wind up considerably the magnitude of the reset response being provided in an attempt by the control to bring the process back to its control point. It will be evident, of course, that when the process again becomes exothermic the reset action which has been accumulated is no longer needed and if not removed will tend to produce overshoot in the temperature of the process as it returns to its control point. In such a situation it is obviously desirable to remove any integral or reset type action which has accumulated in the controller at the time of the process variable under control reaches its set point and it is further desirable that such a removal of the accumulated reset is preferably accomplished without creating a bump in the process. Thus, it is desirable to remove the accumulated integral action without a step change in the process control signal.
Still another process which can advantageously utilize the present invention is the process of controlling the generation of electricity at central stations, particularly in those stations where it is arranged that the control of generation is divided among several generators some of which are assigned the duty of responding to the swings in the required generation while others are assigned the task of dividing the total generation amongst them in such a way as to provide maximum economy in the production of energy from the station. Thus, the economical distribution when made includes all of the output of the stations including any which was initially picked up by the regulating units responding to the fast changes. In such a control system it is desirable that the generation initially picked up by the regulating units should be redistributed amongst the units assigned to provide the economic production of energy without the need for producing a control error in an opposite direction from the one which was corrected by the regulating units in order to remove from the regulating units and assign to the economic units the sustained load which must be carried by the station.